Many H5N1 cases bunched in families, report says

Nov 28, 2005 (CIDRAP News) – More than a third of the human cases of H5N1 avian influenza that occurred over a 19-month period were clustered within families, suggesting the possibility that some family members caught the virus from others, according to a recent report.

Forty-one of 109 cases (38%) identified between January 2004 and July 2005 occurred in 15 families, with between two and five cases per family, according to the report by Sonja J. Olsen and colleagues, published as a letter in the November issue of Emerging Infectious Diseases. Olsen works in Thailand for the International Emerging Infections Program of the US Centers for Disease Control and Prevention.

Most human cases of avian flu are believed to have stemmed from exposure to sick poultry. The virus has not yet shown an ability to spread easily between people, which is seen as the key missing prerequisite for an H5N1 pandemic.

Researchers previously identified one cluster, involving a young Thai girl and her mother and aunt in September 2004, as a probable result of person-to-person transmission. Too little information is available to conclude whether the virus spread from person to person in any of the other families, the new report says.

Family clusters don't necessarily mean the virus is spreading from person to person, the report notes. They may simply mean that relatives were exposed to H5N1-infected poultry at the same time.

However, in three family clusters, all in Vietnam, the first and second patients fell ill more than a week apart, which suggests that they probably didn't acquire the virus from the same source at the same time, the report says.

The authors also found that family clusters were slightly more common in the period from December 2004 through July 2005 than they were in the preceding year: 9 clusters in 243 days, or 3.7 per 100 days, versus 6 clusters in 365 days, or 1.6 per 100 days (relative risk, 2.3; 95% confidence interval, 0.8 to 6.3). The difference was similar when the researchers compared the period from December 2004 through July 2005 with the same 8-month period a year earlier.

Although this increase was not significant, the authors write, "We believe any cluster of cases is of great concern and should be promptly and thoroughly investigated because it might be the first indication of viral mutations resulting in more efficient person-to-person transmission."

The researchers defined a family cluster as two or more family members with laboratory-confirmed H5N1 avian flu or two or more family members with severe pneumonia or respiratory death, when at least one member had confirmed H5N1.

Of the 41 patients in the 15 family clusters, 25 (61%) died, the report says. Another four patients recovered, while three were never sick even though they tested positive for the virus. The outcomes for the other nine patients were unknown.

The article says that family clusters are still occurring, but they "do not appear to be increasing as a proportion of total cases."

In line with previous reports, the authors also note a decline in the overall death rate for human cases: 32 of 44 cases (73%) from December 2003 through November 2004, versus 23 of 65 cases (35%) from December 2004 through July 2005.